The Force Table Vector Addition and Resolution

Transcription

1 Name School Date The Force Table Vector Addition and Resolution Vectors? I don't have any vectors, I'm just a kid. From Flight of the Navigator Explore the Apparatus/Theory We ll use the Force Table Apparatus in this lab activity. Important Note: Working with vectors graphically is very imprecise. You should expect to see lower precision in this lab. But working with vectors graphically will give you a better understanding of vector addition and vector components. Figure 1 The Force Table Apparatus Note: When you re asked to provide a figure you may sketch it; or you may create it with the force table apparatus, print it out and paste it into the space provided. If the printed figure is too large to fit in the space provided, just attach it at the end of the lab. Make sure you label it to match the number of the figure. Ex. The figure above is Figure 1. Lab_ah-Force Table 1 Rev 5/2/12

2 Procedure I. Addition of Two Dimensional Vectors In this part you ll add two forces to find their resultant using experimental addition, graphical addition, and analytical addition. The two forces you ll work with are: F 2 =.300 g N at 320 F 4 =.180 g N at 60 You should know three things about the way these forces are stated from the introductory video. The subscripts refer to the hanger numbers..180 g N is a short hand for.180 kg g. So F 2 =.180 kg 9.81 N/kg = 1.77 N. We ll refer to the forces using the more convenient.180 g N system just for this apparatus..180 g N is the total weight of the hanger system including the hanger itself. So 130 grams were added. A. Experimental Addition By experimental addition we mean that we will actually apply the forces to an object and measure their total, resultant effect. We'll do that by first finding the equilibrant, E, the force that exactly balances them. When this happens the ring will be centered. The resultant, R, is the single force that exactly balances the equilibrant. Enable pulleys 2 and 4 and disable pulleys 1 and 3 using their check boxes. Add masses to hangers 2 and 4 to produce forces F 2 and F 4 and then adjust the angles as specified. Use the purple and blue pointers of the pulley systems, not the strings, to set the angles. You can zoom in to set the angles more precisely. Enable pulley 3. Using pulley 3, find the equilibrant, E to just balance (cancel) F 2 and F 4. You ll need to adjust the mass on the hanger and the angle until the ring is approximately centered on the central pin. You can add and remove masses to home in on it. 1. Experimental Equilibrant, E: g N, From your value of E, what should be the resultant of F 2 and F 4? 2. Predicted Resultant, R: g N, Disable Pulleys 2 and 4 and enable pulley 1. Experimentally determine the resultant by adjusting the mass on pulley 1 and its angle until it balances the equilibrant. That is, the ring should not move substantially when you change between F 2 and F 4, and just F Experimental Resultant, R: g N, B. Graphical Addition Using the default g vector scale, create vector arrows (± 3 grams) for F 2, F 4. For example, drag the purple vector by its body and drop it when its tail (the square end) is near the central pin. It should snap in place. Then drag the head (tip) of the arrow in the direction of F 2 and increase its length until its magnitude is approximately equal to 180 g. Repeat for F 4 with the blue arrow. The resultant, R, is the vector sum of F 2 and F 4. Add F 2 and F 4 graphically by dragging the body of say, F 4 until its tail is over the tip of F 2 and releasing it. Form the resultant, R, by creating an orange vector from the tail of the first (F 2 ), to the head of the last, (F 4 ). This works the same in either order. 1. Graphical Resultant, R: g N, The equilibrant, E should be the same magnitude as R, but in the opposite direction. Produce E with the green vector arrow. 2. Graphical Resultant, E: g N, Lab_ah-Force Table 2 Rev 5/2/12

3 Draw each of the four vector arrows on Figure 2 OR print out and paste your figure over Figure 2. Label each vector with its total force value. Ex. Label the blue F 4 vector.180 g N. Vectors F 2 and F 4 should be shown added in their tail to head arrangement. Figure 2 3: Describe what we mean by the terms resultant and equilibrant in relation to the forces acting in this experiment? 4: From your figures, which two forces, when added would equal zero? Circle two F 2 F 4 E R 5. Show this graphical addition in the space to the right. You ll want to offset them a bit since they would otherwise be on top of one another. Figure 3 6. What three vectors, when added would equal zero? Circle three F 2 F 4 E R 7. Show this graphical addition in the space to the right. 8. Vector E should appear in both Figure 3 and Figure 4. Why? Figure 4 Lab_ah-Force Table 3 Rev 5/2/12

4 Put all four vectors back at the center of the force table with their tails snapped to the central pin. Click the check boxes beside the purple and blue arrows under Show Components. This will give you a visual check on your analytical calculations of the component values in part A3. So be sure and look back at the figure as you do part A3. C. Analytical addition with components 1. You ve found experimentally and graphically how to add the forces F 2 and F 4 to produce the resultant, R. Now let s try analytical addition. Using the table provided, find the components of F 2 and F 4 and add them to find the components of R. Use these components of R to determine the magnitude and direction of R. Note that one of the four components will have a negative sign. The components now displayed on the force table should make it clear why. 2. Show all your calculations leading to your value for R below. Remember, a vector has both a magnitude and a direction. You ll notice that some bits and pieces of the solution have accidentally been left for you to start with. x-components y-components F 2x = g N F 2y = g N F 4x = g N F 4y = g N R x = g N R y = g N R = g N, F 2x = F 2 cos(40 ) =.300 g N cos(40 ) =.230 g N F 4y = F sin(60 ) =.180 g N sin(60 ) =.156 g N R y = g N R 2 = R x 2 + R y 2 tan(θ) = R y/r x θ = II. Simulation of a slackwire problem. Let s model a realistic system similar to what you might find in your homework. Figure 5 shows a crude figure of slackwire walker, Elvira, making her way across the wire. She weighs 450 N. (About 100 lbs.) At a certain instant the two sides of the rope are at the angles shown. Only friction allows her to stay in place. The gravitational force is acting to pull her down the steeper hill. If she was on a unicycle she d tend to roll to the center. It s complicated. The single rope acts like two separate sections of rope in this situation with different tension forces on either side of her. The steeper rope has the greater tension. To understand this it helps to imagine her at the extreme left where the left section of rope is almost vertical and the right one is much less steep. In this case T 3 is providing almost all of the vertical support, while T 1 pulls her a little to the right. It helps to just try it. Attach a string between two objects in the room. Leave a little slack in it. Now pull down at various points. You ll feel the big frictional tug on the more vertical side and less from the more horizontal side. We want to explore this by letting her move along the rope. Lab_ah-Force Table 4 Rev 5/2/12

5 Figure 5 A. Experimental Determination of T 1 and T 2 1. Preliminary prediction: Which tension do you think is greater, T 1 or T 3? Circle one. First send each of your vectors home by clicking on each of their little houses. Then remove all the masses from your four pulleys by clicking in the total mass boxes beside each hanger. Elvira weighs 450 N. We ll let one gram represent 1 N on our force table and set our vector scale to N. (Note the label to the left of each vector.) We ll picture our force table as if it were in a vertical plane with 270 downward and 90 upward. We ll use pulleys 1, 3, and 2 to provide our two tensions, T 1 and T 3, and the weight of Elvira respectively. Turn on those three pulleys and turn off pulley 4 by clicking their check boxes as needed. 2. Prediction: Since θ 3 = 2 θ 1, do you think T 3 will be about twice T 1? For Elvira, move hanger 2 to 270 and set its total mass to 450 g to represent 450 N. Move Pulleys 3 and 1 to match the angles in Figure 5. Adjust the masses on each pulley until you achieve equilibrium. It s best to alternate adding one mass to each side in turn until you get close to equilibrium. Also be sure to set the angles with the green, orange, and purple pointers. You may want to readjust them as needed. 3. T 3 = N at T 1 = N at 10 How d that prediction go (#2)? If trig functions were linear we wouldn t need them. 5. Write a statement about the relationship between the steepness of the rope and the tension in that rope for this vertical arrangement. B. Graphical Check of our Results Using the vector scale of N, create vector arrows for each force, T 3, T 1, and W (Elvira s weight). Draw your three vectors on Figure 6 or print out your force table image and paste it on top of the figure. Figure 6 Lab_ah-Force Table 5 Rev 5/2/12

6 1. How can we graphically check to see if our values for T 1 and T 3 are reasonably correct? How are T 1, T 2, and W related? What would happen if any one of them suddenly went away? Elvira would no longer be in. Thus the three vectors are in equilibrium and must add to equal zero! What would that look like? The sum, resultant, of three vectors is a vector from the tail of the first to the head of the last. If they add up to zero, then the resultant s magnitude would be zero which means that the tip of the final vector would lie at the tail of the first vector. Try it in with your apparatus. Leave T 1 where it is and then drag T 3 s tail to T 1 s head. Then drag W s tail to the head of T What about your new figure says (approximately) that the three forces are in equilibrium? 3. Draw your new figure with the three vectors added together on figure 7 or print out your force table image and paste it on top of the figure. Figure 7 C. Analytical Check of Your Results If our three vectors add up to zero, then what about their components? 1. When you add up the x-components of all three vectors the sum should be 2. When you add up the y-components of all three vectors the sum should be 3. Test your predictions using the following table. Don t forget the direction signs! x-components y-components F 1x = N F 1y = N F 3x = N F 3y = N W x = N W y = N ΣF x = N ΣF y = N 2. Show your calculations of the eight values in the table in the space provided below. Lab_ah-Force Table 6 Rev 5/2/12

7 Before we continue Why all the tension? Why does the tension on each side have to be so much larger than the weight actually being supported? All the extra tension is being supplied by the x-components. Then why not just get rid of it? You d have to move the supports right next to each other to make both ropes vertical. That would be pretty boring and frankly nobody would pay to see such an act, even if they tried calling it Xtreme Urban Slackwire. Similarly, with traffic lights, a huge amount of tension is required to support a fairly light traffic light. The simpler solution of a pair of poles in the middle of the intersection wouldn t go over much better than Xtreme Urban Slackwire. Next time you see large structures supporting large electrical wires look for places where the wire has to change direction. (North to East for example.) The support structures in straight stretches don t have to be really sturdy since they have horizontal forces pulling equally in opposite directions. Thus they just have to support the weight of the wire. The corner structures have to provide these horizontal forces and they have to support it in two different directions. Thus they re much sturdier and larger to give them a wider base. They often have guy wires to the ground to help provide these forces. III. Simulation of a symmetrical slackwire problem. In Figure 8, Elvira has reached the center of the wire. This is where she would be if she rode a unicycle and just let it take her to the bottom. The angle values are just guesses. They d be between the 10 and 20 we had before, but the actual value would depend on the length of the wire and its elastic properties. Figure 8 Without changing the masses, adjust both angles to What does the symmetry of the figure suggest about how the tensions T 1, and T 3 should compare? 2. Similarly what can you say about comparative values of the x-components T 1 x and T 3 x? (Ex. Maybe T 1x = 2 T 3x?) 3. What can you say about comparative values of the y-components T 1y and T 3y? 4. Knowing that the weight being supported is 450 N, what can you say about actual values of the y-components T 1y and T 3y? 5. T 1 y = T 2 y = N You can now easily calculate T 1 and hence T T 1 = T 3 = N at 15 Show the calculation of T 1 below. Lab_ah-Force Table 7 Rev 5/2/12

8 7. Change each T to as close as you can get to this amount. Does this produce equilibrium? 8. With your apparatus, create the two vector arrows to match this amount - one for T 1 and one for T 3. Add T 1 + T 3 + W graphically. Draw or paste a copy of this graphical addition below. 8. Create the two vector arrows to match this amount - one for T1 and one for T3. Add T1 + T3 + W graphically. Draw or paste a copy of this graphical addition in Figure 9. Figure 9 You ll notice that this figure doesn t differ much from the previous one. The graphical tool we re using is not very precise. The same goes for the real world. Measuring the tension in a heavy electrical cable or bridge support cable is very difficult. One good method involves whacking it with a hammer and listening for the note it plays! IV. A Quick look at vector subtraction. Here s the scenario. We have a three-person kinder, gentler tug-o-war. The goal is to reach consensus, stalemate. Two of our contestants are already at work. Darryl 1 = 800 N at 0 Darryl 2 = 650 N at 240 The question is - how hard, and in what direction, must Larry 3 pull to achieve equilibrium? 1. As before, set up pulleys 1 and 2 to represent these forces using 1 gram/1 newton. 2. Create orange and purple vectors to match using the N for your vector scale. Attach them to the central pin. (These won t appear in Figure 10 in their current form.) One method of solving the problem is to add the Daryl forces and complete the triangle to find the resultant. Larry s force would be the equilibrant in the other direction. Here s another way. The following vector equation describes the result when the system is in equilibrium. Σ F = Darryl 1 + Darryl 2 + Larry 3 = 0 We can find Larry 3 by subtracting Darryl 1 and Darryl 2 giving us Larry 3 = Darryl 1 Darryl 2 Lab_ah-Force Table 8 Rev 5/2/12

9 Remember, these are all vectors. We re doing vector subtraction. So how do we subtract them? One way is to interpret the negative signs as indications of direction and then add them in the usual manner tail to head. So Larry 3 = ( Darryl 1 ) + ( Darryl 2 ) Means to draw the vector ( Darryl 1 ) and ( Darryl 2 ) which are the opposites of Darryl 1 and Darryl 2 and add them. That is, we can subtract vectors by adding their negatives. So if Darryl 1 = 800 N at 0 Darryl 2 = 650 N at 240 then the negatives of these are vectors of the same lengths but in the opposite directions. Thus, Darryl 1 = 800 N at 180 Darryl 2 = 650 N at Ruining the hard work you just did, create these two vector arrows radiating from the center of the force table. You ll do this by just reversing the directions of both arrows. Leaving Darryl 1 pointing at 180, add Darryl 2 in the usual tail to head fashion. Larry 3 is the sum of these two. Create the green Larry 3 vector from the tail of Darryl 1 to the head of Darryl 2. (Sorry.) Draw or paste a copy of this graphical addition in Figure 10. Label your three vectors Larry 3, Darryl 1, Darryl 2. Figure Larry 3 (graphical) = N (from the length and direction of your green, Larry 3 vector) Move pulley 3 to the position indicated by the direction of Larry 3. Place the necessary mass on hanger three to produce the Larry 3 force. 5. Larry 3 (experimental) = N Ta Da! I hope this has made vectors as little bit less mystifying. Lab_ah-Force Table 9 Rev 5/2/12

10 This page intentionally left blank. Lab_ah-Force Table 10 Rev 5/2/12